# Lecture 3

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Today
• Thermoresistors
• Power
Resistivity
Resistivity (Ch 25)
For most materials, the resistivity increases
with temperature:
This is true for ohmic materials. Note that
semiconductors are more complex, and
may have resistivities that decrease with
temperature.
Resistivity
Example: Resistance thermometer.
The variation in electrical resistance with
temperature can be used to make precise
temperature measurements. Platinum is
commonly used since it is relatively free from
corrosive effects and has a high melting point.
Suppose at 20.0°C the resistance of a
platinum resistance thermometer is 164.2 Ω.
When placed in a particular solution, the
resistance is 187.4 Ω. What is the temperature
of this solution?
Electric Power (Ch 25)
Power, is the energy transformed by a device
per unit time:
dU = Vdq
or
Electric Power
The unit of power is the watt, W.
For ohmic devices, we can make the
substitutions:
Electric Power
Calculate the resistance of a 40-W
automobile headlight designed for 12 V.
Electric Power
What you pay for on your electric bill is
not power, but energy – the power
consumption multiplied by the time.
We have been measuring energy in
joules, but the electric company
measures it in kilowatt-hours, kWh:
1 kWh = (1000 W)(3600 s) = 3.60 x 106 J.
Electric Power
Example: Electric heater.
An electric heater draws a steady 15.0 A
on a 120-V line.
How much power does it require and how
much does it cost per month (30 days) if it
operates 3.0 h per day and the electric
company charges 9.2 cents per kWh?
Electric Power
Example: Lightning bolt.
Lightning can transfer 109 J of energy across
a potential difference of 5 x 107 V during a
time interval of about 0.2 s.
Use this information to estimate
(a) the total amount of charge transferred
between cloud and ground,
(b) the current in the lightning bolt, and
(c) the average power delivered over the 0.2
s.
Power in Household Circuits
Example: Will a 20A
fuse blow?
Determine the total
current drawn by all
the devices in the
circuit shown.
Power in Household Circuits
Example: A dangerous extension cord.
Your 1800-W portable electric heater is too
far from your desk to warm your feet. Its
cord is too short, so you plug it into an
extension cord rated at 11 A. Why is this
dangerous?
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